Method of and apparatus for producing small particles



Jan. 16, 1968 ECK METHOD OF ANDVAPPARATUS FOR PRODUCING SMALL PARTICLES Filed Dec.

5 ME Em mm NW l 5 ,3 UT E S mm L 5 R E $0" P w w F .P 3 5 a H a 9 9 l 3 4. 3 A v T B "u n E w R 7 I m n 5 v E i F R G E E R R W 3 R R W T 7 m B m 2 A l (T 0 A A H R C C B RI. RI I. ED ,ED V DIN PM W; m WI T T United States Patent Ofiice 3,353,846 Patented Jan. 16, 1968 3,363,846 METHDD OF AND APPARATUS FOR PRODUCING SMALL PARTICLES John E. Ecir, Apoilo, Pa., assignor to Nuclear Materials and Equipment Corporation, Apollo, Pin, a corporation of Pennsyivania Filed Dec. 16, 1965, Ser. No. 514,251 3 Claims. (Cl. 241-15) This invention relates to the art of producing small particles from masses of larger bodies and has particular relationship to producing small particles, highly-reactive materials such as zirconium, hafnium, titanium, niobium and tantalum and their alloys. A typical alloy of zirconium is referred to as Zircaloy alloy; this alloy consists of zirconium with small quantities of such metals as tin, iron, chromium, nickel. A typical Zircaloy alloy consists of zirconium, 1.5% tin, .2% iron and .1% chromium. Such materials have substantial ductility at normal temperatures.

Usually it is required that the particles produced have dimensions lying within specified limits. Typically, it is desirable that the particles have dimensions between about 44 microns and 177 microns; that is, +325 to -80 mesh. Such particles would be selected by screens which would reject particles greater than 80 mesh and by seiving which would pass particles smaller than 325 mesh.

Another condition imposed On the final particled prod net is that the oxygen and nitrogen, whether present in the form of gases occluded in the product or present in the form of oxides or nitrides, be minimized. Other gases such as hydrogen should also be minimized. Typically, the starting mass which is particled may contain about 40 parts per million nitrogen and 1000 ppm. oxygen; it is required that the particled product contain no more than 70 ppm. nitrogen and 1500 ppm. oxygen.

A further condition imposed on the final particled product is that the product have a substantial bulk density; flaky material is undesirable.

It is an object of this invention to produce particled products of the above listed materials, which shall meet the above conditions; that is, products whose particles shall have dimensions reliably lying between predetermined limits, which shall contain a minimum of such elements as hydrogen, oxygen, and nitrogen, and which shall have substantial bulk density.

Typically, the particles are produced by breaking up the starting material in a high-speed hammer mill. For clarity, the units of the material subjected to the forces of the hammer mill will be referred to herein as bodies and the units of the product of the mill as particles. In the typical practice of this invention the starting bodies are in the form of chips or trimmings.

The contamination by nitrogen, oxygen and hydrogen of particle masses of the above-listed metals and alloys is not readily avoided because those metals are prone to interstitial contamination by these gases during the attrition or grinding or breaking up of these materials. In accordance with the teachings of the prior art the grinding takes place in inert gaseous atmospheres. To reduce the contamination the material is repeatedly passed through hammer mills in these atmospheres. But at the temperatures at which the grinding takes the materials are ductile and tend to resist size reduction demanding large energy per unit mass which in turn increases the ductility. This process is inefficient, the inetliciency manifesting itself principally in increase in the temperature of the product being ground. The high temperature results in reaction of the metal with the residual gases present or absorbed in the material. Attempts to achieve the highest purity results in balling-up of the particles.

Attempts have been made in accordance with the teachings of the prior art, to carry out the grinding in solid carbon dioxide (carbon dioxide snow). But it has been found that this results in excessive oxygen. It has been realized, in arriving at this invention that the excessive oxygen is produced in the CO snow grinding because the heat developed at the points of impact of the hammer mill blades with the bodies decomposes the carbon dioxide into carbon monoxide and oxygen and the oxygen appears as contamination in the particled product.

In accordance with this invention the grinding is carried out in liquid nitrogen. In accordance with the broader aspects of this invention the grinding may also be carried out in liquid argon. Specifically, the grinding is carried out in successive stages by a plurality of hammer mills and the liquid nitrogen is supplied to the mass being ground in one or more of the stages in controlled quantities through an orifice at each stage. Nitrogen has a boiling point of about -320 F. and a high heat of vaporization, about 78.5 B.t.u. per pound. It has been discovered that by grinding the mass in the presence of the liquid nitrogen, the liquid nitrogen in vaporizing is highly effective in maintaining the material being ground in the region in which the grinding takes place at the desired low temperature where this grinding is effective and reaction between nitrogen and the material being ground does not take place.

To illustrate the effectiveness of the liquid nitrogen, the following parameters of grinding apparatus may be assumed:

50 pounds of zirconium is to be ground per hour. The material is ground in three hammer mills each of 1 horsepower outputor 2545 B.t.u. per hour. The temperature is to be maintained at 300 F. The overall efficiency of thermal conversion and heat leakage is 50%.

The heat which must be absorbed to reduce 50 pounds of zirconium to 300 F. is the heat which must be absorbed per pound, 26 B.t.u., multiplied by 50.

(50) (26) :1300 Btu.

The heat developed by the mills which must be absorbed is 2545 3=7635 B.t.u.

Total heat necessary in the operation is the sum of these or 8935 B.t.u.

Since the efiiciency is 50%, the total heat required is 2X 8935:17870 Btu.

It may be assumed that the nitrogen is converted into vapor at -300 F. absorbing 78.5 B.t.u. per pound. The number of pounds of liquid nitrogen required is then or 4.55 per pound of material ground. The actual temperature of the material being ground is not --300 F. but a higher temperature which is still so low that no reaction takes place between the nitrogen and the metal, and the nitrogen is not absorbed materially in the particled product.

The actual temperature is governed by the requirement that the bulk density of the particled product shall be substantial. It has been found that by setting the temperature of the grinding by proper selection of the orifices controlling the flow of nitrogen at each mill, the desired bulk density is achieved. The chips which constitute the starting material are generally needle shaped and tend to break through the small dimension under the impact from the blades. The tendency of this repeated breaking is to produce fiat bodies or flakes and only flakes would be produced if the temperature is maintained so low that the material being ground has substantially no ductility,

and is not deformed (bent over) by some blows from the blades rather than being broken. To achieve the vestigal ductility needed, the temperature of the material being ground must be set on the basis of observation of the product by selecting the appropriate orifice opening at each stage. For this purpose it is not necessary that the precise temperature of the mass being ground be known. A temperature representative of the actual temperature of the mass at each stage is measured. Preliminary operations are then carried out and the particled product and its behavior examined. By adjusting the orifices the temperature of the mass may be increased if the product is flaky or decreased if the product ballsup or is excessively contaminated. A resulting relative temperature measurement is observed. The actual producing operation is maintained such that the relative temperature reading is at the magnitude for which the product during the preliminary operation had the desired properties.

The nitrogen vapor produced during the process is permitted to escape through the available openings in the apparatus. Since vapor is produced at a high rate, it prevents atmospheric air from penetrating into the mass being ground.

For a better understanding of this invention, both as to its organization and as to its operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection With the accompanying drawing, in which:

FIGURE 1 is a diagrammatic view in side elevation showing a preferred embodiment of this invention; and

FIG. 2 is a fragmental diagrammatic view in front elevation showing one of the hammer mills of this embodiment with the related apparatus.

The apparatus shown in the drawing includes a hammer mill 11 of three stages 13, 15 and 17. The mass of bodies to be particled is delivered to the input stage 13 by a vibrator 19 to which it is supplied through a hopper 21. Each stage 13, 15, 17 includes a hammer mill 31, the blades 33 of which are driven at a high speed by a motor 35. The blades 33 are of a material which retains reasonable ductility at the temperature of liquid nitrogen. The material to be particled is fed into the blades through a channel 37 terminating in a receiving hopper.39. The particled material is derived through a screen 41. Typically, the screen 41 of the stages 13, 15, 17 may have a mesh such as to reject bodies exceeding 80 mesh. The material is collected in a container 36 and seived to reject particles typically smaller than 325 mesh in dimension.

The apparatus also includes a source 51 of liquid nitrogen and a manifold 53. The nitrogen is supplied as a liquid to the manifold 53 through a valve 55 which sets the pressure. A gauge 57 is provided for measuring the pressure. Typically the pressure of the nitrogen is 30 to 40 pounds per square inch. The manifold 53 is provided with outlet tubes 61 each of which extends into a stage 13, 15, 17 and terminates in an orifice 63 near the center of the shaft 65 for the associated blades 33, (FIG. 2). Orifices 63 of different dimensions may be provided on the ends of the tubes 61. Generally the orifices for the lower stages 15 and 17 may be of smaller diameter than the orifices for the upper stage 13 and 15 respectively because a portion of the liquid nitrogen flows or drips down from an upper stage to a succeeding lower stage. Typically the dimensions of the orifices may be .088 inch for 13, .052 inch for 15 and .040 inch for 17.

A representative relative temperature of each stage is measured by a thermocouple 71 which extends into the product of the stage and is connected to a meter 73. The temperature of each stage may be controlled by controlling the fiow of nitrogen to the stage. By adjusting the temperature a dense product with a minimum contamination may be achieved effectively and efficiently.

t The meters 73 enable the operator to maintain the desired temperature. The pressure built up by the evaporating nitrogen serves as a shielding atmosphere to prevent contamination by air.

The following summary will aid the understanding of the invention:

Certain metal powders such as zirconium, titanium, hafnium, niobium, and tantalum are prone to interstitial contamination by nitrogen, hydrogen, and oxygen during mechanical attrition of the massive metal. Typically, it is imperative that the product be ground to sizes finer than about 177 microns (US. Standard 80 mesh) and that the product be, to the highest practical level, free of liquid and gaseous contamination. The prior-art procedure is to grind these metals in protective inert gaseous atmospheres such as helium or argon. Because these metals are ductile and atheir ductility increases with temperature they cannot be crushed effectively by any of the numerous devices available for size reduction of relatively brittle materials.

Moderate success has been achieved in reducing metal turnings of the aforementioned ductile metals (zirconium, titanium, hafnium, niobium and tantalum) to fine powders in inert atmospheres, by repeatedly passing these in chip form through high-speed hammer mills. However, because of the ductility of these metals, the process is relatively inefficient in comparison to size reduction of brittle materials. Large amounts of energy per unit mass are required because the principal amount of energy applied by the mill is consumed in deforming rather than in fracturing the particles. This absorption of deformation energy results in increased particle temperature, which further increases the ductility and process inefficiency. Further, the increased temperature makes the metals more susceptible to contamination by virtue of their increased reaction rate with gaseous impurities present in the grinding atmosphere. If atmosphere purities are controlled to the ultimate, another process problem arises. This problem exhibits itself as reconsolidation or clumping or balling-up of finer particles into massive balls or lumps and pasting of the clumps on the grinder surfaces. This occurs because of the combination of excessive temperature, high impact energy and the availability of freshly cut clean surfaces and their high susceptibility to impact or pressure welding.

This invention is a process and apparatus to perform this manufacturing operation so as to permit increased energy input per unit mass, without comprising the powder metal product purity and to achieve improved grinding efficiency.

In using liquid nitrogen or liquid argon, as a coolant directly injected into the product being ground, it has been found that product quality, production rate and grinding is superior to the conditions typical of the prior art method which utilized argon or helium. In addition equipment wear is reduced and there is complete freedom from pressure welding and reconsolidation of the material being ground. Further, there is no tendency of the particles to paste themselves into the grinder working surfaces.

The performance of the grinding specifically in nitrogen but broadly in either nitrogen or argon or both yields the following improvements to the process.

(1) The material being ground at low temperature has a greatly reduced aflinity or reaction rate for pick-up of contaminant gases. This permits the use of cheaper protective atmosphere gas such as nitrogen in preference to argon or helium; the nitrogen is in itself a serious contaminant in the case of higher-temperature grinding 7 but is not a contaminant at the low temperature. In addition the protective atmospheres gas being used may be of reduced purity because of the greatly reduced affinity of the material at the low temperature for the contaminant gases oxygen and hydrogen.

(2) Some metals have reduced ductility with a decrease in temperature. They also generally have significantly increased notch sensitivity to breakage and many metals pass through radical transformations from ductile to brittle in the temperature region below room temperature. The utilization of low temperature grinding takes advantage of Whatever reduction of ductility and increased notch sensitivity to fracture that may be available for the particular metal to improve the mechanical efficiency of the grinding process.

(3) Cold welding is principally a function of pressure, temperature, and availability of atomically clean surface area. Grinding in liquid nitrogen permits application of increased mechanical energy rates without traversing the threshold of temperature and pressure for cold welding.

(4) The maintenance of low temperatures in liquid nitrogen due to greatly reduced reaction rates and mobility by diffusion mechanisms limits the diffusion rates of interstitial contaminants to surface layers. The aforementioned reactive metals require special treatment in grinding. Because they have high afi'inities for oxygen and nitrogen and consequently they cannot exist bare in normal air without immediately undergoing adsorption of surface reaction layers of oxides or nitrides. Consequently, when such metals are exposed to outside air the minimum possible contamination is that amount corresponding to this surface layer. By use of this liquid nitrogen powder production process, these minimal surface layers can be immediately and safely satisfied from the grinding atmosphere and with assurement that diffusion will not extend contaminant adsorption significantly beyond the surface film. The safe application of these films minimizes the pyrophoricity of fine ground powders of these reactive metals.

The successful placement of these surface films on the particles immediately limits the probability of clean metal surfaces being placed intimately in contact, thereby minimizing the probability of cold welding of the particles to each other or to the grinder working surfaces.

(5) The combination of these advantages yields improvement in eflieiency, quality and safety to the product.

While preferred embodiments of this invention have been disclosed herein many modifications thereof are feasible. This invention then is not to be restricted except insofar as is necessitated by the spirit of the prior art.

I claim as my invention:

1. The method of producing small particles from a mass of larger bodies of materials such as zirconium, hafnium, titanium, niobium and tantalum and their alloys while maintaining the hydrogen, oxygen and nitrogen in the ground product at a minimum, the said method comprising subjecting said bodies to mechanical forces and continuously injecting liquid nitrogen into said mass as said mass is being so subjected to said mechanical forces.

2. The method of claim 1 for producing particles having a mesh of less than 80 wherein the particles are ground in a plurality of successive stages and liquid nitrogen is injected into the masses being subject to mechanical forces at least in certain of said stages.

3. The method of claim 1 wherein, for this purpose controlling the properties of the small particles produced, the temperatures at which the bodies are subject to mechanical forces takes place is controlled by controlling the quantity of nitrogen injected.

References Cited UNITED STATES PATENTS 2,583,697 1/1952 Hendry 99-23 2,609,150 9/1952 Bludeau 241-15 2,618,018 11/1952 Downing 260232 2,659,986 11/1953 Hink 34-5 2,665,850 1/1954 Wiczer 260 2,892,697 6/1959 Davies 75--.5 2,895,689 7/1959 Edwards 241-23 2,919,862 l/196O Beike 241-23 3,072,347 1/1963 Dombrowski 241-3 HARRISON L. HINSON, Primary Examiner. 

1. THE METHOD OF PRODUCING SMALL PARTICLES FROM A MASS OF LARGER BODIES OF MATERIALS SUCH AS ZIRCONIUM, HAFNIUM, TITANIUM, NIOBIUM AND TANTALUM AND THEIR ALLOYS WHILE MAINTAINING THE HYDROGEN, OXYGEN AND NITROGEN IN THE GROUND PRODUCT AT A MINIMUM, THE SAID METHOD COMPRISING SUBJECTING SAID BODIES TO MECHANICAL FORCES AND CONTINUOUSLY INJECTING LIQUID NITROGEN INTO SAID MASS AS SAID MASS IS BEING SO SUBJECTED TO SAID MECHANICAL FORCES. 